The use of radar probes for the determination of the profile of the loading surface of a shaft furnace has been known for some time and is described, e.g. in U.S. Pat. No. 4,744,010, the disclosure of which is incorporated by reference. Radar probes have, in effect, several important advantages with respect to mechanical probes, the most important of which is that of speed of measurement which allows, e.g. the measurement of several points or the carrying out of a scanning in order to determine the profile of the loading surface, contrary to the most of the mechanical probes which can only carry out spot measurements. Unfortunately, these radar probes had, up to the present time, the handicap of being much more expensive than mechanical probes. However, the reduction in the price of electronic components is making radar probes more and more competitive.
Among radar probes two different types are essentially distinguished, namely the so-called monostatic probes, i.e., those which have a single antenna serving as both a transmitter and receiver of radar waves and the so-called bistatic probes which comprise two separate antennas, one serving as a transmitting antenna and the other serving as a receiving antenna for the reflected radar waves. Each type of probe has its very specific advantages and disadvantages. Thus, for example, monostatic probes must comprise means, such as a magnetic core circulator, for separating the received signal from the transmitted signal, whose amplitude is generally larger. The major disadvantage of monostatic probes, particularly in continuous mode, i.e., non-pulsed, is the poor capability of separating the transmitted signal from the received signal. The advantage of the monostatic probe is its mechanical simplicity and a larger antenna aperture, i.e., a more sensitive gain for a given aperture.
The advantage of a bistatic probe is that it allows a better separation between the transmitted signal and the received signal. On the other hand, the disadvantage of bistatic antennas is the oblique path of the radar beam between the transmitter and the receiver, i.e., the angle of reflection. Given that this angle of reflection increases as the distance to be measured decreases, bistatic probes are less suitable for measuring short distances.